1. Field of the Invention
The present invention relates to a circuit for separating a composite video signal into a chrominance signal and a luminance signal in a color television system. More particularly, the present invention relates to a motion adaptive luminance and chrominance signal separating circuit which separates a composite video signal into a chrominance signal and a luminance signal by adaptively comb-filtering the composite video signal in response to a motion degree of an image displayed in a television receiver.
2. Description of the Prior Art
It is generally known that the human eye distinguish chrominance data such as hue, saturation, etc., with great detail, unlike luminance data. Thus, most television video signal assign a wide band (4.2 MHz in the NTSC system) to luminance data. In contrast, the resolution for hue and saturation only requires a 1/10 to 1/3 bandwidth of brightness.
According to the aforementioned characteristic, the chrominance signals have a much narrower bandwidth than the luminance signals. For instance, in the NTSC system. an I signal component of the chrominance signal has a band of 1.3 MHz, and a Q component has a band of 0.5MHz, respectively.
In the NTSC system, relative to the frequency of the carrier of the complete video signal the color subcarrier is at about 3.58 MHz, the luminance signal extends to 4.2 MHz. the Q component of the modulated chrominance signal extends 0.5 MHz to each side of the subcarrier, and the sidebands of the I component of the modulated chrominance signal extend from 1.5 MHz below to 0.6 MHz above the subcarrier. The subcarrier wave itself is suppressed in the transmitted video signal.
Formation of the transmitted video signal from the original RGB signals from a video camera is referred to encoding, and recovery of the RGB signals from the received video signal at the receiver is referred to decoding. The decoding includes an operation of separating a received composite video signal into a chrominance signal and a luminance signal, which has most commonly been effected by comb-filtering.
FIG. 1 is a block diagram for showing a conventional comb-filter.
In FIG. 1, the circuit includes an input terminal 11, a 1 H (a period of horizontal synchronizing signal; hereinafter, referred to as H)-delaying circuit 12, a first subtracter 13, a 1/2-amplifier 14, a band pass filter (hereinafter, referred to as "BPF") 15, and a second subtracter 16.
In the circuit as shown in FIG. 1, when a composite video signal is inputted to the 1 H-delaying circuit 12 which has passed through the input terminal 11, the 1 H-delaying circuit 12 delays the composite video signal for one horizontal scanning period and outputs the 1 H-delayed composite video signal to the first subtracter 13. The first subtracter 13 subtracts the 1 H-delayed composite video signal from a currently composite video signal to generate a subtracted signal, and outputs the subtracted signal which has a component of a chrominance signal to the 1/2-amplifier 14. Then, the 1/2-amplifier 14 1/2-amplifies the subtracted signal from the first subtracter 13 to generate a 1/2-amplified signal, and outputs the 1/2-amplified signal to the BPF 15. The BPF 15 filters the 1/2-amplified signal from 1/2-amplifier 14 to generate a filtered signal, and outputs the filtered signal as a chrominance signal C to an exterior. The second subtracter 16 subtracts the chrominance signal C from the currently composite video signal to generate a luminance signal Y, and outputs the luminance signal Y to the exterior.
Therefore, in the comb-filter 10, as the 1 H-delayed composite video signal is subtracted from a currently composite signal on the basis of the fact that chrominance signals between two horizontal lines have the phase difference of 180.degree., a luminance signal is removed from a composite video signal which is inputted through the input terminal 11, so that a chrominance signal is continuously obtained.
But, the comb-filter 10 has problems, such as a hanging dot phenomenon and a dot crawl phenomenon in a screen of television receiver. The hanging dot phenomenon is generated when chrominance signals are transient into the vertical direction, and the dot crawl phenomenon is generated when chrominance signals and luminance signals are mixed while being passed through a band pass filter.
For removing the problems such as the hanging dot and dot crawl phenomena, several methods have been presented. A correlation adaptive comb-filtering method is one of the methods.
FIG. 2 is a block diagram of a conventional correlation adaptive comb-filter.
In FIG. 2, the correlation adaptive comb-filter 20 includes an input terminal 21, a first 1H-delaying circuit 22, a second 1H-delaying circuit 23, a first subtracter 24, a second subtracter 25, a first 1/2-amplifier 26, a second 1/2-amplifier 27, a first LPF 28, a second LPF 29, a comparator 30, and a data selector 31.
In the correlation adaptive comb-filter 20, the first 1 H-delaying circuit 22 delays a composite video signal passed through the input terminal 21 for a horizontal line scanning time to generate a 1 H-delayed signal, and outputs the 1 H-delayed signal to the second 1 H-delaying circuit 23, the first subtracter 24, and the second subtracter 25, respectively. The second 1 H-delaying circuit 23 delays the 1 H delayed signal from the first 1 H-delaying circuit 22 for the horizontal line scanning time to generate a 2 H-delayed signal, and outputs the 2 H-delayed signal to the second adder 25. The first subtracter 24 subtracts a currently composite video signal through the input terminal 21 from the 1 H-delayed signal to generate a first subtracted signal 33, and outputs the first subtracted signal 33 to the first 1/2-amplifier 26 and the first LPF 28. Also, the second subtracter 25 subtracts the 2 H-delayed signal from the 1 H-delayed signal to generate a second subtracted signal 34, and outputs the second subtracted signal 34 to the second 1/2-amplifier 27 and the second LPF 29, respectively. Each of the first and second 1/2-amplifier 26 and 27 1/2-amplifies the first subtracted signal 33 and the second subtracted signal 34, and outputs the signals 1/2-amplified by each of them to a first input terminal A and a second input terminal B of the data selector 31, respectively.
Each of the first LPF 28 and the second LPF 29 filters the first subtracted signal 33 and the second subtracted signal 34, and outputs a first filtered signal 35 and a second filtered signal 36, which are generated by respectively filtering the first subtracted signal 33 and the second subtracted signal 34, to a first input terminal A and a second input terminal B of the comparator 30. The comparator 30 compares the first filtered signal 35 with the second filtered signal 36, thereby generating a control signal according to a result of the comparison, and provides the control signal to a control signal input terminal S of the data selector 31 so that the data selector 31 may selectively output one of the signals 1/2-amplified by each of the first 1/2-amplifier 26 and the second 1/2-amplifiers 27 as a chrominance signal C.
For example, when an amplitude of the first filtered signal 35 is higher than an amplitude of the second filtered signal 36, the comparator 30 generates a high logic signal as the control signal, and provides the high logic signal to the data selector 31. Then, the data selector 31 selects the first subtracted signal 33 1/2-amplified by the first 1/2-amplifier 26, and outputs the selected signal as the chrominance signal C to an exterior. On the contrary, when the amplitude of the first subtracted signal is lower than the amplitude of the second subtracted signal, the comparator 30 generate a low logic signal as the control signal, and provides the low logic signal to the data selector 31. Then, the data selector 31 selects the second subtracted signal 34 1/2-amplified by the second 1/2-amplifier 27, and outputs the selected signal as the chrominance signal C to the exterior.
Further, the chrominance signal C from the data selector 31 is filtered by a BPF, such as in FIG. 1, and the filtered chrominance signal is subtracted from a composite video signal, thereby obtaining a luminance signal.
Therefore, according to the comb-filter 20, defaults which are displayed on a screen of TV, such as the hanging dot and dot crawl phenomena, can be prevented by respectively separating a composite video signal into a chrominance and luminance signal in response to the degrees of correlation between composite video signals of three horizontal scanning lines.
However, when a degree of the correlation between scanning lines is low, the comb-filter 20 of FIG. 2 still causes the hanging dot and dot crawl phenomena because the comb-filter 20 only separates a composite video signal into a chrominance and a luminance signal according to a degree of the correlation between three horizontal scanning lines.
U.S. Pat. No. 5,225,899 (issued to Young-June Park on Jul. 6, 1993) discloses one example of a circuit for separating a composite video signal into a luminance and a chrominance signal according to the correlation of the image to be displayed in TV. The circuit disclosed in U.S. Pat. No. 5,225,899 includes a line comb-filtering circuit for comb-filtering the input composite video signal, a line comb-filter for detecting each vertical correlation from the comb-data outputted from the line comb-filtering circuit and selecting one of the comb-data according to the detected vertical correlation, a horizontal band pass filter circuit for inversely and reinversely delaying the video signal supplied from the line comb-filter and producing the chrominance signal data and the vertical correlation data from the video signals, a horizontal correlation adaptive circuit for detecting a horizontal correlation from the horizontal correlation data and selecting one of the chrominance signal data according to the detected horizontal correlation, and a luminance and chrominance signal output portion for receiving the video signal and chrominance signal supplied from the line comb-filtering circuit and a horizontal correlation adaptive circuit, respectively, to output the finally separated luminance signal and chrominance signal. The circuit prevents the hanging dot and dot crawl phenomena, thereby improving the image quality.
However, in the circuit, when the correlation of vertical and horizontal direction is small, and an image displayed on the TV screen is still, the hanging dot and the dot crawl phenomena are still generated on the TV screen.
For the foregoing reasons, there is a need for a circuit that can remove the hanging dot and dot crawl phenomena.